Ordinarily, the colorimetric value of a colored material changes with temperature, and this phenomenon is known as “thermochromism”. Thermochromism is a problem whenever a very precise colorimetry is desired, since the error due to the temperature of the object to be measured affects the colorimetric value.
It is known that the temperature variation caused by this error is substantially linear over temperature and is reversible. FIG. 10 shows the result of analyzing a color chart (referred to in the following as “patch”) printed with an electrophotographic printer, by measuring its spectral reflectivity. As can be seen in FIG. 10, the color fluctuations can be classified into fluctuations caused by the influence of fluorescent substance, such as fluorescent brighteners contained in the media, and fluctuations caused by the influence of non-fluorescent substances, such as toner compositions. Furthermore, as shown in FIG. 11, the spectral reflectivity of fluorescent substances changes in the vertical direction of the chart depending on the temperature, and as shown in FIG. 12, the spectral reflectivity of non-fluorescent substances changes in the horizontal direction of the chart depending on the temperature.
In order to increase their whiteness, some media that are used for ordinary printing contain fluorescent brighteners that absorb UV light and exhibit fluorescence in the visible part of the spectrum. The fluorescent brightening effect due to the fluorescent brighteners depends on the temperature, that is, increases or decreases with the temperature, so that also the colorimetric value of the printed matter that is obtained by printing on the media containing the fluorescent brighteners will depend on and change with the temperature.
The following is an explanation of a color stabilization process and a color matching process performed by an ordinary printing apparatus. Conventionally, in order to output a desired color with a printing apparatus, such as an electrophotographic printer, a color conversion look-up table (referred to below as “LUT”) is used. Among color conversion LUTs, there are LUTs used for a calibration in order to keep the printing apparatus in a given constant state and LUTs used for color matching, for which ICC profiles are a typical example. To prepare these color conversion LUTs, a plurality of color charts, such as IT8.7/3 patches or the like, are output with the printing apparatus. Using a colorimeter, such as a spectrophotometer, the colors of the plurality of color charts are measured, thus obtaining their colorimetric values, and by associating the device value with the device independent value, a color conversion LUT is prepared. By using the prepared LUT, differences in color among devices as well as differences in color due to variations among print engines can be reduced, and better color matching and color stabilization can be attained.
Recently, such LUTs are generated in real-time by using a color sensor mounted inside the electrophotographic printer. Such a printer detects, with the built-in color sensor, the patch immediately after fixing, and returns the colorimetric value of the patch to the LUT generation process.
However, in ordinary electrophotographic printers, patches that have just been is fixed are very hot (about 70 degrees Celsius), so that the colorimetric value of the patches changes due to the above-mentioned thermochromism. Therefore, there is the problem that a LUT that is suitable for the environment in which the user views the printed matter (room temperature) cannot be prepared with high precision.
Consequently, there is a need for a technology in printing apparatuses to correct the colorimetric value in view of the temperature at the time of the colorimetry. For example, as shown in FIG. 13, printing apparatuses have been proposed, which prepare the LUT considering the media temperature immediately after the fixing of the patch (see Japanese Patent Number 3555706, for example). The printer 1 shown in FIG. 13 is characterized in that it generates colorimetric values for any desired temperature by performing a correction regarding temperature fluctuations to the colorimetric values of any patch printed on a medium (any color reproduced with the printer), using a colorimetric value correction unit 114.
FIG. 14 shows an outline of the operation of the colorimetric value correction unit 114. As shown in this figure, the device value 1112 of the patch (patch data), whose color is to be measured, is entered from a color matching unit 111 into the colorimetric value correction unit 114. Furthermore, the patch temperature 1221 and the colorimetric value 1231, which are output from a temperature sensor unit and a color sensor unit within the engine, are entered into the colorimetric value correction unit 114. Then, using the specified object temperature and a temperature characteristics LUT 1141 of a reference patch that has been set in advance, the colorimetric value correction unit 114 corrects the colorimetric value and outputs the corrected colorimetric value 1142.
However, in ordinary electrophotographic printers, including the above-described conventional printers performing temperature-dependent correction of the colorimetric values, all patches keep a certain temperature immediately after fixing, but when the sheet paper has been transported for a certain distance after the patch has been fixed, the temperature differs from patch to patch. The reason for this is that the amount of toner differs from patch to patch, so that the specific heat differs from patch to patch and there are differences in the amount by which the temperature drops.
Therefore, in order to make the colorimetric value correction for the temperature of the patch more accurate, it is necessary to measure the temperature of the patch under test at the same time of colorimetry, for each colorimetric analysis using a radiation thermometer or the like on each patch to be measured, so that the system becomes more complicated and more expensive.
Thus, since the patch temperature immediately after fixing the patch changes, it is difficult to perform a suitable colorimetric value correction taking into account the patch temperature.